This invention relates to an improved process for pyridine base synthesis, and to a particular class of shape-selective zeolite catalysts for use in the same which have been advantageously modified by treatment with one or more compounds containing tungsten, zinc or tin.
The term "base synthesis" is known and used in the pyridine field and in this application to identify a process by which bases of pyridine or alkylpyridine derivatives are prepared by reacting aldehydes and/or ketones with ammonia in the gas phase using a heterogeneous catalyst. Some examples of base synthesis reactions (and their common names where appropriate) include: the synthesis of pyridine and beta-picoline from acetaldehyde and formaldehyde (the "pyridine-beta reaction"); the synthesis of alpha- and gamma-picoline from acetaldehyde (the "alpha-gamma reaction"); the synthesis of 2,6-dimethylpyridine ("2,6-lutidine") from acetone and formaldehyde; the synthesis of 2,4,6-trimethylpyridine ("sym-collidine") from acetone alone or with acetaldehyde; the synthesis of pyridine and beta-picoline from acrolein alone or with acetaldehyde; the synthesis of 3,5-dimethylpyridine from propionaldehyde and formaldehyde; and the synthesis of beta-picoline from acetaldehyde, formaldehyde and propionaldehyde. Many others are known and reported or practiced in the art, and are equally considered within the scope of the description and invention herein.
The catalysts used in these pyridine base synthesis reactions have varied from alumina which was used early-on either alone or as a support for zinc fluoride or other metal salts to an amorphous structure incorporating both silica and alumina which became an important commercial catalyst. See U.S. Pat. Nos. 2,807,618 and 2,744,904; and German Patent No. 1,255,661. Similarly, the reactor designs for these heterogeneous gas-phase reactions have varied within the basic categories of fixed-bed and fluid-bed forms. The advantages of fluidized beds were recognized early-on (see U.S. Pat. No. 2,807,618) as evidenced by the fact that the handful of commercial-scale base synthesis units operating today worldwide all incorporate fluidized catalyst beds. One reason for this is that base synthesis reactions always produce deposits of dark, mostly carbonaceous materials referred to as "coke" which tend to foul the catalyst thereby gradually reducing its activity. Although variations are observed, all catalysts accumulate these coke deposits at a appreciable rate such that periodic action is required. As discarding catalyst is not desirable for economic reasons, regeneration by heating in air or other oxygen-containing gases is commonly employed. This regeneration/combustion process is very exothermic and also best carried out in a fluid bed Process. C. L. Thomas, "Catalytic Processes and Proven Catalysts", Academic Press. N.Y., pp. 11-14 (1970).
Accordingly, a common technique has long been to run two fluid beds concurrently, one for reaction and one for regeneration,. with catalyst continuously or intermittently cycled between the beds. Operating parameters such as circulation rates, contact times, temperatures and the like are readily determined by skilled operators in view of the specific reactions and/or ingredients used. See, e.g., German Patent No. 2,203,384. An ancillary benefit of this technique is that product yields from base synthesis reactions carried out in fluidized beds are recognized to be generally higher than in corresponding fixed-bed reactions. This was emphasized in two families of patents issued to BP Chemicals U.K. Ltd. of London, England, one for alpha-gamma synthesis (British Patent No. 1,188,891; German Patent 1,903,879; and Canadian Patent No. 852 745) and the other for pyridine-beta synthesis (British Patent No. 1,235,390; Canadian Patent No. 851,727; and German Patent No. 1,903,878). These BP patents, and German Patent No. 1,903,878 in particular, compare fixed- and fluid-bed reactions using catalysts of amorphous silica-alumina or of metal compounds such as the oxides or fluorides of lead, zinc and cadmium on amorphous silica-alumina supports.
This same advantage of fluid-bed usage was reported by Feitler et al. in U.S. Pat. No. 4,675,410 for base synthesis catalysts composed of shape-selective aluminosilicates (commonly referred to as "zeolites") used in their acidic form. These crystalline zeolites had earlier been reported for base synthesis reactions by Chang et al. in U.S. Pat. No. 4,220,783 both in their acid- or H-form and as ion-exchanged with cadmium, copper or nickel. Several examples in the Chang patent demonstrated deactivation of the catalyst over time thereby also suggesting the desirability of a fluid-bed to reactivate the catalyst by heating in air in any commercial application.
In general, these base synthesis reactions have received universal acceptance as evidenced by their continuous commercial use for many years. The products of base synthesis, including pyridine, alpha-, beta- and gamma-picoline, nearly all the lutidines, and primarily the symmetrical isomer of collidine, have all shown commercial importance in the world chemical market albeit of varying values and volume requirements. See Goe, "Pyridine and Pyridine Derivatives," Encyclopedia of Chemical Technology, Vol. 19, 3rd. Ed. (1982). It is also the case that improvement in the yields of these reactions and variation in their product ratios may be desirable according to market trends for such pyridine-derivative products as the herbicide paraquat, vitamins such as niacin and niacinamide, tire cord adhesive derived from 2-vinylpyridine, the tuberculosis drug Isoniazid, and so forth. One approach to this end has examined variations in reaction conditions such as temperature, velocity or contact time, mole ratios of feed stocks, and the like. Here, optimization of yield or product ratio is generally accomplished by known techniques employed by those skilled in this area. A second approach has involved catalyst variation in which far less predictability exists.
For example, while work early-on was with amorphous silica-alumina or other catalysts, the concentration in recent years has shifted to these so-called shape-selective zeolites which are aluminosilicates of definite crystal structure having activities and pores of size similar to that of other commercially-interesting molecules. See, e.g., E. G. Derouane. "New Aspects of Molecular Shape-Selectivity: Catalysis by Zeolite ZSM-5", Catalysis by Zeolites, ed. B. Imelik et al., Elsevier, Amsterdam, pp. 5-18 (1980). These materials are often defined by a constraint index which is an experimentally-derived number based on the observed relative rates of reaction of straight and branched-chain molecules. Frillette et al., J. Catal., 67, 218 (1981). The term "zeolite" has even acquired a broader meaning in the art, and is accordingly used in this application to mean more than the original crystalline aluminosilicate materials. For example, "zeolite" is understood and meant to also include compositions such as gallosilicates, ferrosilicates, chromosilicates and borosilicates. Crystalline aluminum phosphates ("ALPO's") and silicon-aluminum phosphates ("SALPO's") are also included in its coverage because of their catalytic ability, as is even theoretically-pure crystalline silicalite such as a S-115 material marketed by Union Carbide Corporation of N.Y.
In these zeolite materials, some ion-exchange properties are generally thought to exist due to positive ions associated with the trivalent molecular centers (e.g., aluminum, boron. gallium, etc.) that are present in the network of tetravalent silicon centers. Although ALPO is an exception to this, and silicalite may be an exception but for residual aluminum in its crystal structure, it has also been thought that catalytic activity is associated somehow with these ion-exchange sites. As synthesized, zeolites typically have sodium or quaternary ammonium ions in their crystal structures. If these ions are exchanged for ammonium (NH.sub.4 +) ions and the resulting ammonium zeolite heated, an acidic or "H-form" zeolite results with these acid centers believed to be associated with some catalytic activity. For instance, an H-form of ZSM-5 zeolite is marketed by The Mobil Corporation of N.Y. and is used in the synthesis of gasoline from methanol.
One approach at optimizing yield and/or product ratios from base synthesis reactions has been to stress maximizing these acidic sites. For example, the Feitler patent claims the specific benefit of a higher ratio of pyridine in the pyridine-beta synthesis by use of a zeolite catalyst of preferably 80-100% this H-form although no direct comparison with other ion-exchanged zeolites is reported.
Other positive ions have also been exchanged for the sodium, ammonium or H-sites in the zeolite structure. For example, cracking catalysts have used a rare earth ion-exchanged form of the large-pore zeolite Y (called "REY"). C. L. Thomas, "Catalytic Processes and Proven Catalysts", supra, pp. 30-31. Precious metals have been exchanged in both large-pore and shape-selective zeolites to produce reforming catalysts. E.G. Derouane, "New Aspects of Molecular Shape-Selectivity: Catalysis by Zeolite ZSM-5", supra, p. 17. The Chang patent also reported use of zeolites ion-exchanged with cadmium, copper or nickel ions in addition to the H-form of Mobil's ZSM-5 material in base synthesis reactions. The Chang patent did test the catalytic activity of these metal ion-forms, but did not speculate on whether they existed solely or survived in their ionic state or were reduced to base metals.
More recently, Shimizu et al. described base synthesis reactions using shape-selective zeolites treated with thallium, lead or cobalt ions or compounds in an European application, Serial No. 232,182 published Aug. 12, 1987. These metals were ion-exchanged into a zeolite of alkali, ammonium or acid form in an aqueous medium or were mixed in the solid state with no apparent effect from the mode of mixing used. As the Feitler patent, Shimizu also reported the desire for a low yield of beta-picoline from pyridine-beta base synthesis. However, this work does not permit direct comparison with Feitler as Shimizu used a reaction mixture with so little formaldehyde (0.5 moles per mole of acetaldehyde) that it necessarily produced about equal amounts of beta- and gamma-picoline which are of questionable commercial utility. See, e.g., Beschke, Ullmann Encyclopedia, p. 593 (1980).
It is in the light of this background, and of the large body of general chemical literature concerned with base synthesis processes (see F. Brody and P. R. Ruby. Pyridine and Its Derivatives, E. Klingsberg ed., Vol. 1 (1960); N. S. Boodman et al., Ibid, Supplement Abramovitch ed., Vol. 1 (1975); T. D. Bailey, G. L. Goe and E. F. V. Scriven, Ibid. Supplement G. R. Newkome ed., Vol. 5 (1984)), that the applicants approached this study with the objectives of providing at least equivalent overall yields and, where appropriate, the selectivity to vary product ratios within reason to meet varying economic conditions.